An automobile with an internal combustion engine using gasoline or heavy oil generally has a serious influence on the generation of pollution like atmospheric pollution. Therefore, in order to reduce the generation of pollution, there have been many efforts to develop a hybrid vehicle or an electric vehicle.
Recently, there has developed a high power secondary battery using a high energy density non-aqueous electrolyte. The high power secondary battery may be provided in plural and connected in series in order to form a high capacity secondary battery.
As described above, the high capacity secondary battery (hereinafter, called “battery”) is typically comprised of the plurality of secondary batteries connected in series. In case of the battery, particularly, an HEV battery, since a few or a few ten secondary batteries are alternately charged and discharged, there is a necessity of managing the battery to control the charging and discharging of the battery and maintain the battery in an appropriate operation state.
To this end, there is provided BMS (Battery Management System) for managing all the states of the battery. The BMS detects voltage, current, temperature or the like, estimates SOC through a calculating operation and controls the SOC so as to optimize the fuel consumption efficiency of a vehicle. In order to precisely control the SOC, it is necessary to exactly measure the SOC of the battery in the charging and discharging operations are carried out.
As a prior art, there is disclosed Korean Patent Application No. 2005-0061123 (filed on Jul. 7, 2005) entitled “Method for resetting SOC of secondary battery module”.
In order to precisely calculate the SOC of the battery, the above-mentioned prior art includes measuring a current value, a voltage value and a temperature vale of a battery module when turning on a switch, calculating initial SOC using the measured values, accumulating the current value, calculating actual SOC according to the accumulated current value, determining whether the battery module is in a no-load state, determining whether the actual SOC is within a setup range that can be measured by accumulating the current value if the battery module is in the no-load state, and calculating the SOC according to the voltage value by measuring the voltage value if the actual SOC is outside the setup range. However, the prior art does not disclose a method that applies a simple equivalent circuit to an actual battery and an apparatus thereof.
Generally, SOCi does not have errors in the short term, but, as shown in FIG. 1, there is a tendency that the errors are accumulated. Therefore, in case that the battery is operated for a long time, considerable error is occurred. Especially, the accumulative error is mostly generated when the charging or discharging of battery is completely achieved. This is caused by that the degree of precision is influenced by the errors occurred by reduction of SOC due to self-discharge, and omission of LBS digit of CPU for calculating the SOC. Further, since the precision degree of the SOC is largely dependent on a current measuring sensor, it is impossible to correct the errors when the sensor has a trouble.
However, as shown in FIG. 2, SOCv measures the SOC through an open circuit voltage. In this measuring method, it is possible to obtain very precise results when a current is not flowed. However, when the current is flowed, the precision degree of the SOCv is dependent on a charging and discharging pattern of a battery. Therefore, since the precision degree of the SOC is also dependent on the charging and discharging pattern, it is deteriorated. Furthermore, the charging and discharging pattern that deteriorates the precision degree of the SOCv is within a range that a typical battery is used. Thus, although only the SOCv is used, there is also a problem that has to accept the considerable error.